Carbon nanotubes (CNTs; sometimes referred to as buckytubes) are allotropes of carbon with a cylindrical nanostructure. These cylindrical carbon molecules have novel electrical, mechanical, thermal and chemical properties that make them useful in applications such as nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors.
Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs. The ends of a nanotube might be capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while they can be up to 18 centimeters in length. Iijima, S. Nature 354, 56-58 (1991), Sumio Iijima & Toshinari Ichihashi, Nature 363, 603-605 (17 Jun. 1993), Wang et al., Nano Letters 9 (9): 3137-3141 (2009). Nanotubes are often categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) depending on the number of layers of the tube.
The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamonds, provides the molecules with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals forces.
Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. Single-walled nanotubes are an important variety of carbon nanotube because they exhibit electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. In particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior, whereas MWNTs are zero-gap metals. Single-walled nanotubes are the most likely candidate for miniaturizing electronics beyond the micro electromechanical scale currently used in electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors. One useful application of SWNTs is in the development of the first intramolecular field effect transistors (FET). Production of the first intramolecular logic gate using SWNT FETs has recently become possible as well. To create a logic gate you must have both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs within the same molecule.
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. There are two models which can be used to describe the structures of multi-walled nanotubes. In the so called “Russian Doll” model, sheets of graphite are arranged in concentric cylinders, e.g. a (0,8) single-walled nanotube (SWNT) within a larger (0,10) single-walled nanotube. In the so called “Parchment model,” a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å.
The properties of double-walled carbon nanotubes (DWNT) are similar to SWNT their resistance to chemicals can be significantly improved. This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the carbon nanotubes. In the case of SWNT, covalent functionalization will break some C═C double bonds, leaving “holes” in the structure on the nanotube and thus modifying both its mechanical and electrical properties.
Carbon nanotubes can be used in electronics applications, for example in transistors, light emitters for display, and flexible electrodes. Their possible use as electrical conductors has not been as extensively investigated. The instant disclosure demonstrates growth and patterning as well as integration of carbon nanotubes as an electrical conductor in interconnect structures. The advantages of carbon nanotube-based interconnects include high current carrying capability, no electromigration, robust mechanical properties, and high thermal conductivity.